RE: XAUI AC coupling
Rich,
No, I am not abstaining. I will make it brief and clear: I vote for
mandatory AC coupling.
Thanks,
Vipul
===============================
> -----Original Message-----
> From: owner-stds-802-3-hssg@xxxxxxxx
> [mailto:owner-stds-802-3-hssg@xxxxxxxx]On Behalf Of Rich Taborek
> Sent: Saturday, September 23, 2000 11:43 AM
> To: HSSG
> Subject: Re: XAUI AC coupling
>
>
>
> Vipul,
>
> I'd like to set the record straight: Neither I nor anyone else involve
> in this thread has ever recommended mandatory DC-coupling. The P802.3ae
> approved XAUI baseline, developed initially by the Hari group which
> included experts from Ethernet, Fibre Channel and InfiniBand, ended up
> requiring AC-coupling. I have proposed removing this requirement and
> allowing either AC or DC-coupling. My reading of your first paragraph
> suggests that we are in violent agreement. If this is indeed the case,
> then the only remaining decision is whether AC and DC-coupling of XAUI
> is described in the standard or not. I'm happy going either
> way on this.
> I favor leaving the details to the implementer.
>
> In your second paragraph you seen to waver from violent agreement and
> note a very specific application of XAUI as a system ASIC to
> transceiver
> module link. I maintain that this specific application is well outside
> the scope of the standard and only representative of one of a myriad of
> applications for XAUI. For example, a simple early 10GE XAUI
> application
> is to couple the XAUI output directly to a laser diver and
> post-amplifier set of a WWDM module. The XAUI interface is short, the
> laser driver to XAUI interface is likely to be custom, and DC-coupling
> is appropriate. As I have pointed out in prior notes, a prevalent XAUI
> application will be as a fixed chip-to-chip interconnect not involving
> optical modules at all. It is a straightforward implementation
> detail to
> select either AC or DC-coupling in the latter scenario. The standard
> should not dictate sub-optimal implementations.
>
> Vipul, I can't seem to place you in either the "Mandatory AC-coupling"
> or "Allowable AC or DC-coupling" categories. From your last note it
> seems like you're abstaining. I'd also like to get other's perspective
> on this issue.
>
> Best Regards,
> Rich
>
> --
>
> Vipul Bhatt wrote:
> >
> > Rich,
> >
> > Let's see if we can refine the question, in the hope of
> making progress.
> >
> > We do have some common ground: If a PHY module is going to
> be purchased
> > by a switch manufacturer, it will likely end up as a pluggable or
> > solderable module, at the XAUI interface. To manage the multiple
> > buyer-supplier scenarios, it is best to use AC coupling. If,
> however, a
> > PHY module is going to be integrated by the switch manufacturer on a
> > single board, then XAUI becomes the switch manufacturer's
> internal design
> > responsibility, and they should have the freedom to choose DC or AC
> > coupling. Just as it would be wrong to burden a pluggable module with
> > mandated DC coupling, it would be wrong to burden an
> integrated single
> > board design with mandated AC coupling.
> >
> > Beyond this common ground, where we go from here becomes an
> interesting
> > choice. One approach would be to leave the coupling issue to the
> > implementers. Another approach would be to say, the popular
> purpose of
> > XAUI is to allow easy separation of switch and PHY module
> > responsibilities, and to allow the implementation of pluggable or
> > solderable PHY modules. To help achieve that purpose in a
> bullet-proof
> > fashion, we should mandate AC coupling. This is the
> "majority gets its
> > way" approach. At the moment, I favor the second approach. To make
> > further progress, it will help to know what others think.
> >
> > Regards,
> > Vipul
> >
> > vipul.bhatt@xxxxxxxxxxx
> > (408)542-4113
> >
> > ===============
> >
> > > -----Original Message-----
> > > From: owner-stds-802-3-hssg@xxxxxxxx
> > > [mailto:owner-stds-802-3-hssg@xxxxxxxx]On Behalf Of Rich Taborek
> > > Sent: Saturday, September 23, 2000 12:55 AM
> > > Cc: HSSG
> > > Subject: Re: XAUI AC coupling
> > >
> > >
> > >
> > > Vipul,
> > >
> > > OK, I asked for it... Now I'm forced to respond.
> > >
> > > A PHY module to ASIC connection, with the ASIC being in-line and
> > > eventually connected to a switch fabric, is very likely to
> be a XAUI
> > > implementation. If the module is pluggable, then it is
> very likely that
> > > the XAUI link would be AC-coupled to insure maximum
> interoperability.
> > > However, if the module is fixed, there are non negligible cost,
> > > reliability and performance advantages to employing
> DC-coupling instead
> > > if applicable. There is no risk. My point all along, is that
> > > coupling is an implementation detail and should not be a standard
> > > mandate.
> > >
> > > Your PECL example reflects information an implementer
> should be able to
> > > glean from a manufacturers data sheet. Many XAUI devices
> will be fixed.
> > > The implementer simply reads the relevant data sheets for both XAUI
> > > devices and determines whether or not AC-coupling is required. If
> > > AC-coupling is not required, the implementer may choose to
> DC-couple.
> > > The determination of signal coupling requirements is
> standard practice
> > > for chip-to-chip interconnects.
> > >
> > > Your third paragraph seems to describe a scenario where an
> implementer
> > > makes a bad decision to employ DC-coupling where the
> device specs for
> > > the two XAUI devices employed in the link dictated
> AC-coupling. I was
> > > unaware that the purpose of the standard was to force suboptimal
> > > implementations in case an implementer misinterprets device
> > > data sheets.
> > >
> > > I believe that most implementers would be incensed by such imposing
> > > regulations. I certainly hope that the same implementer
> doesn't rely on
> > > the standard for all other aspects of XAUI link
> implementation, such as
> > > power supply decoupling, trace layout, connector choice,
> via design,
> > > etc. to insure that their XAUI links work reliably.
> > >
> > > I don't understand the relevance of LVDS to this discussion, please
> > > explain.
> > >
> > > I agree that if either an implementer is uncertain about
> DC bias or DC
> > > bias itself is uncertain, that AC-coupling should be used.
> However, you
> > > seem to be describing a scenario where too much
> uncertainty exists. It
> > > is highly likely that the operation of the XAUI link will
> be uncertain
> > > in this case.
> > >
> > > To conclude, your assumed XAUI configuration is system ASIC to
> > > transceiver module which exemplifies only one possible
> XAUI application
> > > and one in which DC-coupling is applicable and preferred in many
> > > instances. In addition your desire is to impose suboptimal
> > > implementations on all XAUI links in case an implementer
> > > happens to make a mistake. I have to respectfully disagree
> that either
> > > argument dictates that XAUI AC-coupling is technically required.
> > >
> > > Best Regards,
> > > Rich
>
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